1. Field of the Invention
This invention relates generally to a protective relaying system and more particularly to a protective relaying system provided with a digital processing unit for a relay calculation unit.
2. Description of the Prior Art
Hitherto, protective relays have been classified, according to operation principle, into an electromechanical type relay, a static type relay, and a digital type relay.
The electromechanical type relay drives a movable portion by means of flux of magnetic force or electromagnetic force, and opens and closes output contacts by movement of the movable portion.
The static type relay comprises a transistor circuit wherein comparisons of electrical quantities with one another in terms of magnitude and phases thereof are made, and produces an output in accordance with the thus compared result.
The digital type relay has such functions as to sample, with certain specified intervals, electrical quantities indicative of voltages and currents derived from respective phases of a power system to be protected, to hold the respective sampled values, to sequentially rearrange the same by means of a multiplexer, to convert the rearranged electrical quantities into digital quantities by utilizing an analog-to-digital converter, and to process these digital quantities within a digital processing unit on the basis of predetermined relay calculation programs.
The static type relay, most commonly used of the three, will be described with reference to the accompanying drawings.
FIG. 1 is a diagram of principle structure illustrating a static-type mho characteristic distance relay (hereinafter, simply called a mho relay) in a single phase. In FIG. 1, a potential transformer PT and a current transformer CT installed at the terminal of a transmission line TL to be protected transform voltage and current of the transmission line TL, and respectively output a voltage V and a current I in the secondary thereof. Here, V and I indicate a voltage vector and a current vector, respectively. The voltage V and the current I are respectively inputted to an input conversion unit 100, wherein the inputted electrical quantities are converted into various electrical quantities such as IZ, V.sub.p and V required for mho relay calculation, and outputted to a relay-operation judging unit 200.
A vector synthesizing circuit AD in the input conversion unit 100 multiplies the inputted current I by a simulated impedance Z which simulates the line impedance of the transmission line TL so as to output an electrical quantity IZ. Here, the phase of the electrical quantity IZ leads the phase of the current I by a line impedance angle (an angle .angle.Z). ME represents a storage circuit which receives the voltage V and outputs an electrical quantity V.sub.p as a polar quantity. The storage circuit ME, which is comprised of, for example, a series resonance circuit, produces the electrical quantity V.sub.p of damped oscillation so that even in case the input voltage abruptly becomes zero due to occurrence of extremely close fault, the system can maintain a proper direction judgement for a period of a few cycles.
Next, in the relay-operation judging unit 200, a comparator 1 receives the electrical quantity IZ at the positive input terminal, and the electrical quantity V, at the negative input terminal, respectively. The comparator 1 outputs a logical state "1" when an instantaneous value of (IZ-V) is greater than zero, and when smaller than zero, then outputs a logical state "0", respectively. A comparator 2 similarly receives the electrical quantity V.sub.P at the positive input terminal, and a DC reference electrical quantity such as zero volts, at the negative input terminal, respectively. The DC reference electrical quantity is necessarily limited to zero volts, and it may contain a certain amount of DC bias quantity so as to prevent misoperations of the relay-operation judging unit 200 when no signal is inputted thereto. The logical state output signals from the comparators 1 and 2 are inputted to an AND circuit 3, wherein an overlapped angle .theta. (overlapped time) of the phases of the inputted signals is detected, and a logical state signal corresponding to the overlapped angle .theta. is outputted. A time-measuring circuit 4, which is comprised of an ON-delay timer, outputs a logical state "1" when the period during which the logical state signal "1" outputted from the AND circuit 3 remains is greater than a predetermined period, such as a period corresponding to an electrical angle of 90.degree.. The logical state output "1" of the time-measuring circuit 4 indicates that the mho relay is operative, while "0", the mho relay is inoperative. The logical state output signal is used to trip a circuit breaker CB provided in the transmission line TL.
FIG. 2 shows a characteristic diagram of the relay shown in FIG. 1, which indicates that when an angle .theta. defined by the electrical quantities (IZ-V) and V.sub.P is less than 90.degree., that is, when the head of the electrical quantity V lies within the circular area, the relay is operated.
The foregoing description has been made for a single phase of the mho relay, however, for protection of a three-phase circuit, three relays of identical principle and rating should be utilized.
In the case of the mho relay, when an operative region is attempted to extend, a load impedance caused by tidal current can possibly penetrate the operative region of the relay.
In such case, an ohm-characteristic distance relay (hereinafter, simply called an ohm relay) which functions as a blinder is utilized together with the mho relay so as to prevent the ultimate output from being produced in the relay system.
FIG. 3 shows a characteristic diagram of the ohm relay, which indicates that when an angle .theta. defined by the electrical quantities (IZr-V) and IZ.sub.R is less than 90.degree., the relay is to be operated. Here, the principle structure diagram of the ohm relay is not shown, however, it is equal to that of the mho relay shown in FIG. 1 except that the comparators 1 and 2 receive the input electrical quantities IZr and IZ.sub.R instead of the input electrical quantities IZ and V.sub.P, respectively.
In a conventional static-type protective relay system, mho relays and ohm relays are individually constituted by separate circuits, and such relays are required to be installed on respective phases, thus, the system has become inevitably bulky in scale, and also involved in disadvantages such as inefficient maintenance, inspections and cost thereof.